Compliant bipolar micro device transfer head with silicon electrodes

ABSTRACT

A compliant bipolar micro device transfer head array and method of forming a compliant bipolar micro device transfer array from an SOI substrate are described. In an embodiment, a compliant bipolar micro device transfer head array includes a base substrate and a patterned silicon layer over the base substrate. The patterned silicon layer may include first and second silicon interconnects, and first and second arrays of silicon electrodes electrically connected with the first and second silicon interconnects and deflectable into one or more cavities between the base substrate and the silicon electrodes.

RELATED APPLICATIONS

The present application is a continuation of Ser. No. 14/221,071, filedMar. 20, 2014, which is a continuation of U.S. patent application Ser.No. 14/063,963, filed Oct. 25, 2013, now U.S. Pat. No. 8,716,767, whichis a continuation of U.S. patent application Ser. No. 13/543,680, filedon Jul. 6, 2012, now U.S. Pat. No. 8,569,115, which is incorporatedherein by reference.

BACKGROUND

1. Field

The present invention relates to micro devices. More particularlyembodiments of the present invention relate to a compliant bipolar microdevice transfer head and a method of transferring one or more microdevices to a receiving substrate.

2. Background Information

Integration and packaging issues are one of the main obstacles for thecommercialization of micro devices such as radio frequency (RF)microelectromechanical systems (MEMS) microswitches, light-emittingdiode (LED) display systems, and MEMS or quartz-based oscillators.

Traditional technologies for transferring of devices include transfer bywafer bonding from a transfer wafer to a receiving wafer. One suchimplementation is “direct printing” involving one bonding step of anarray of devices from a transfer wafer to a receiving wafer, followed byremoval of the transfer wafer. Another such implementation is “transferprinting” involving two bonding/de-bonding steps. In transfer printing atransfer wafer may pick up an array of devices from a donor wafer, andthen bond the array of devices to a receiving wafer, followed by removalof the transfer wafer.

Some printing process variations have been developed where a device canbe selectively bonded and de-bonded during the transfer process. In bothtraditional and variations of the direct printing and transfer printingtechnologies, the transfer wafer is de-bonded from a device afterbonding the device to the receiving wafer. In addition, the entiretransfer wafer with the array of devices is involved in the transferprocess.

SUMMARY OF THE INVENTION

A compliant bipolar micro device transfer head and head array, and amethod of transferring one or more micro devices to a receivingsubstrate are disclosed. For example, the receiving substrate may be,but is not limited to, a display substrate, a lighting substrate, asubstrate with functional devices such as transistors or integratedcircuits (ICs), or a substrate with metal redistribution lines.

In an embodiment, a compliant bipolar micro device transfer head arrayincludes a base substrate and a patterned silicon layer over the basesubstrate. For example, the base substrate may be a (100) bulk siliconsubstrate. The patterned silicon layer includes a first siliconinterconnect, a first array of silicon electrodes electrically connectedwith the silicon interconnect, a second silicon interconnect, and asecond array of silicon electrodes electrically connected with thesecond silicon interconnect. Each silicon electrode in the first andsecond arrays of silicon electrodes includes an electrode lead and amesa structure that protrudes above the first and second siliconinterconnects. The first and second arrays of silicon electrodes arealigned as an array of bipolar silicon electrode pairs and electricallyinsulated from one another. The first and second silicon interconnectsmay be parallel to each other. Each silicon electrode is alsodeflectable into a cavity between the base substrate and the siliconelectrode. For example, one or more cavities may be formed in the basesubstrate. In an embodiment, the first and second arrays of siliconelectrodes are deflectable into the same cavity in the base substrate.In such an embodiment, the array of bipolar silicon electrode pairs isdeflectable into the same cavity in the base substrate. The cavity mayalso wrap around an end of one, or both, of the first and second siliconelectrodes. In an embodiment, each bipolar silicon electrode pair in thearray of bipolar electrode pairs is deflectable into a separate cavity.A dielectric layer such as a silicon oxide, hafnium oxide, aluminumoxide, or tantalum oxide, covers a top surface of each mesa structure. Aburied oxide layer may be formed between the patterned silicon layer andthe base substrate.

In an embodiment, an array of bipolar silicon electrode pairs form anarray of supported beams spanning between the silicon interconnect andthe second silicon interconnect. For example, an array of oxide jointsmay be formed between the first and second arrays of silicon electrodes.The patterned silicon layer may be on and in direct contact with aburied oxide layer, with the oxide joints on and in direct contact withthe buried oxide layer. The oxide joints may be parallel orperpendicular to the first and second arrays of silicon interconnects,and between the mesa structures of the first and second arrays ofsilicon electrodes. The supported beams may also include bends, forexample, in the silicon electrode leads of the silicon electrodes. Thearray of oxide joints may separate the first and second arrays ofsilicon electrodes along a longitudinal length or a transverse width ofthe array of supported beams.

In an embodiment, an array of bipolar silicon electrode pairs form anarray of cantilever beams spanning between the silicon interconnect andthe second silicon interconnect. In an embodiment, each siliconelectrode in the bipolar silicon electrode pairs is a separatecantilever beam, and an open space is between the mesa structures of thefirst and second arrays of silicon electrodes. The cantilever beams mayinclude bends. In an embodiment, the mesa structures of the first andsecond arrays of silicon electrodes are not separated by an open space.For example, an array of oxide joints may be formed between first andsecond arrays of silicon electrodes for the array of cantilever beams.The patterned silicon layer may be on and in direct contact with aburied oxide layer, with the oxide joints on and in direct contact withthe buried oxide layer. In an embodiment, the oxide joints separate thefirst and second arrays of silicon electrodes along a longitudinallength of the array of cantilever beams. In an embodiment, the oxidejoints are parallel to the first and second silicon interconnects, andare between the mesa structures of the first and second arrays ofsilicon electrodes.

In an embodiment, a buried silicon oxide layer is between the patternedsilicon layer and the base substrate. A first via extends through thebase substrate and the buried silicon oxide layer from a backside of thebase substrate to the patterned silicon layer and, and in electricalconnection with the first silicon interconnect and the first array ofsilicon electrodes. A second via extends through the base substrate andthe buried silicon oxide layer from a backside of the base substrate tothe patterned silicon layer and, and in electrical connection with thesecond silicon interconnect and the second array of silicon electrodes.The vias may extend through the patterned silicon layer or terminate ata bottom surface of the patterned silicon layer.

The dielectric layer covering a top surface of each mesa structure inthe array and the second array may be formed of a material such assilicon oxide, hafnium oxide, aluminum oxide, and tantalum oxide. Insome embodiments, a first dielectric layer is laterally between the mesastructures of the array of silicon electrodes and the second array ofsilicon electrodes in a bipolar electrode configuration, and underneaththe dielectric layer covering the top surface of each mesa structure inthe array and the second array. The dielectric layer may have a higherdielectric constant or dielectric breakdown strength than the firstdielectric layer.

In an embodiment, an method of forming a compliant bipolar micro devicetransfer head array includes etching a top silicon layer of asilicon-on-insulator stack to form an a first array of siliconelectrodes electrically connected with a first silicon interconnect, anda second array of silicon electrodes aligned with the first array ofsilicon electrodes and electrically connected with a second siliconinterconnect to form an array of bipolar silicon electrode pairs, witheach silicon electrode in the first and second arrays of siliconelectrodes including an electrode lead and a mesa structure thatprotrudes above the first and second silicon interconnects. A dielectriclayer is then formed over the first and second arrays of siliconelectrodes, and one or more cavities are etched into the base substratedirectly underneath the first and second arrays of silicon electrodessuch that each silicon electrode in the first and second arrays ofsilicon electrodes is deflectable into the one or more cavities. Etchingof the one or more cavities may be accomplished, for example, with afluorinated plasma of SF₆ or XeF₂. In an embodiment, a separate cavityis etched in the base substrate directly underneath each bipolar siliconelectrode pair. In an embodiment, a single cavity is etched in the basesubstrate directly underneath the array of bipolar silicon electrodepairs. In an embodiment, the single cavity is etching in the basesubstrate so that it wraps around one, or both, of the first and secondsilicon interconnects.

Etching of the top silicon layer may expose a buried oxide layer.Formation of the dielectric layer may be accomplished with a variety oftechniques. In some embodiments, the dielectric layer includes thermaloxidation of the array of silicon electrodes. In some embodiments, apatterned layer is formed over the buried oxide layer and the dielectriclayer after forming the dielectric layer, and using the patterned layerthe buried oxide layer is etched to expose a portion of the basesubstrate. The dielectric layer can be used as an etching mask whenetching one or more cavities in the base substrate directly underneaththe first and second arrays of silicon electrodes.

In an embodiment, an array of joint trenches are etched between the mesastructures of the first and second arrays of silicon electrodessimultaneously with etching the top silicon layer of thesilicon-on-insulator stack to form the first and second arrays ofsilicon electrodes. The dielectric layer may also be formed within thearray of joint trenches and in direct contact with the buried oxidelayer simultaneously with forming the dielectric layer over the firstand second arrays of silicon electrodes. For example, the dielectriclayer may be formed by thermal oxidation of the first and second arraysof silicon electrodes. The dielectric layer may also completely fill thearray of joint trenches with the dielectric layer to form an array ofoxide joints between the first and second arrays of silicon electrodes.

A first backside via opening may be etched through the base substratedirectly underneath the first silicon interconnect, and second backsidevia opening may be etched through the base substrate directly underneaththe second silicon interconnect, and a passivation layer may be formedwithin the first and second backside via openings. In an embodiment, thepassivation layer is formed by thermally oxidizing the base substratewithin the first and second backside via openings simultaneously withthermally oxidizing array of first and second arrays of siliconelectrodes to form the dielectric layer. A patterned conductive layermay be formed within the first and second via openings to makeelectrical contact with the first and second silicon interconnects, forexample, by deposition through a shadow mask.

In an embodiment, the dielectric layer is etched to expose a portion ofthe first and second silicon interconnects simultaneously with etchingthrough the buried oxide layer to expose the portion of the basesubstrate. A first topside via opening is then etched through the firstexposed portion of the first silicon interconnect and the buried oxidelayer, and a second topside via opening is etched through the secondexposed portion of the second silicon interconnect and the buried oxidelayer. A patterned conductive layer can then be formed within the firstand second topside via openings to make electrical contact with thefirst and second silicon interconnects.

In an embodiment, the dielectric layer is etched to expose each of themesa structures simultaneously with etching through the buried oxidelayer to expose the portion of the base substrate. A second dielectriclayer can then be formed over each of the mesa structures. In anembodiment, this may be accomplished by blanket deposition of the seconddielectric layer followed by removal of a portion of the seconddielectric layer. In some embodiments, blanket deposition may beaccomplished by atomic layer deposition. In an embodiment, thedielectric layer may be additionally etched to expose a portion of thefirst and second silicon interconnects, followed by etching a firsttopside via opening through the exposed portion of the first siliconinterconnect and the buried oxide layer, etching a second topside viaopening through the exposed portion of the second silicon interconnectand the buried oxide layer, and forming a patterned conductive layerwithin the first and second topside via openings to make electricalcontact with the silicon interconnect and second silicon interconnect.The second dielectric layer formed over each of the mesa structures andthe conductive layer formed within the first and second topside viaopenings may also be used as an etching mask when etching the one ormore cavities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view illustration of a compliant bipolar micro devicetransfer head array of single sided clamped cantilever beam pairs withno joint in accordance with an embodiment of the invention.

FIG. 1B is a plan view illustration of a compliant bipolar micro devicetransfer head with a pair of single sided clamped cantilever beams andno joint in accordance with an embodiment of the invention.

FIG. 1C is a cross-sectional side view illustration taken alongtransverse line C-C of the compliant bipolar micro device transfer headillustrated in FIG. 1B in accordance with an embodiment of theinvention.

FIG. 1D is a cross-sectional side view illustration taken alonglongitudinal line D-D of the compliant bipolar micro device transferhead illustrated in FIG. 1B in accordance with an embodiment of theinvention.

FIGS. 2A-2B are combination plan view and combination cross-section sideview illustrations taken along lines V-V, W-W, X-X, Y-Y, and Z-Z fromFIG. 1A illustrating a compliant bipolar micro device transfer headincluding an open joint trench between the pair of silicon electrodes,and backside via openings in accordance with an embodiment of theinvention.

FIGS. 3A-3B are combination plan view and combination cross-sectionalside view illustrations of a compliant bipolar micro device transferhead including a double sided clamped supported beam and an oxide jointbetween and connecting the pair of silicon electrodes, and topside andbackside via openings in accordance with an embodiment of the invention.

FIGS. 4A-4B are combination plan view and combination cross-sectionalside view illustrations of a compliant bipolar micro device transferhead including a double sided clamped supported beam and depositeddielectric layer, an oxide joint 119 between and connecting the pair ofsilicon electrodes 110, and topside and backside via openings inaccordance with an embodiment of the invention.

FIGS. 5A-15B illustrate a method of forming a compliant bipolar microdevice transfer head including an open joint trench between the pair ofsilicon electrodes, and backside via openings in accordance with anembodiment of the invention.

FIG. 16A is a plan view illustration of a compliant bipolar micro devicetransfer head array of double sided clamped supported beams and mesajoints in accordance with an embodiment of the invention.

FIG. 16B is a plan view illustration of a compliant bipolar micro devicetransfer head with a double sided clamped supported beam and mesa jointin accordance with an embodiment of the invention.

FIG. 16C is a cross-sectional side view illustration taken alongtransverse line C-C of the compliant bipolar micro device transfer headillustrated in FIG. 16B in accordance with an embodiment of theinvention.

FIG. 16D is a cross-sectional side view illustration taken alonglongitudinal line D-D of the compliant bipolar micro device transferhead illustrated in FIG. 16B in accordance with an embodiment of theinvention.

FIGS. 17A-24B illustrate a method of forming a compliant bipolar microdevice transfer head including a double sided clamped supported beam andan oxide joint between and connecting the pair of silicon electrodes,and topside and backside via openings in accordance with an embodimentof the invention.

FIGS. 25A-30B illustrate a method of forming a compliant bipolar microdevice transfer head including a double sided clamped supported beam anda deposited dielectric layer, an oxide joint between and connecting thepair of silicon electrodes, and topside and backside via openings inaccordance with an embodiment of the invention.

FIG. 31 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with cantilever beam and continuous joint in accordancewith an embodiment of the invention.

FIG. 32 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with cantilever beam and mesa joint in accordance with anembodiment of the invention.

FIG. 33 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with double sided clamped beam and continuous joint inaccordance with an embodiment of the invention.

FIG. 34 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with a double sided clamped beam including a pair ofsilicon electrodes with double bends and a mesa joint in accordance withan embodiment of the invention.

FIG. 35 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with a double sided clamped beam including a pair ofsilicon electrodes with single bends and a mesa joint in accordance withan embodiment of the invention.

FIG. 36 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with a double sided clamped beam including a pair ofsilicon electrodes with double bends and a mesa joint in accordance withan embodiment of the invention.

FIG. 37 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with a double sided clamped beam including a pair ofsilicon electrodes with double bends and a mesa joint in accordance withan embodiment of the invention.

FIG. 38 is a flow chart illustrating a method of picking up andtransferring an array of micro devices from a carrier substrate to areceiving substrate in accordance with an embodiment of the invention.

FIG. 39 is a cross-sectional side view illustration of an array ofcompliant bipolar micro device transfer heads positioned over an arrayof micro devices on a carrier substrate in accordance with an embodimentof the invention.

FIG. 40 is a cross-sectional side view illustration of an array ofcompliant bipolar micro device transfer heads in contact with an arrayof micro devices in accordance with an embodiment of the invention.

FIG. 41 is a cross-sectional side view illustration of an array ofcompliant bipolar micro device transfer heads picking up an array ofmicro devices in accordance with an embodiment of the invention.

FIG. 42 is a cross-sectional side view illustration of an array of microdevices released onto a receiving substrate in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention describe a compliant bipolar microdevice transfer head and head array, and method of transferring a microdevice and an array of micro devices to a receiving substrate. Forexample, the compliant bipolar micro device transfer head and head arraymay be used to transfer micro devices such as, but not limited to,diodes, LEDs, transistors, ICs, and MEMS from a carrier substrate to areceiving substrate such as, but is not limited to, a display substrate,a lighting substrate, a substrate with functional devices such astransistors or integrated circuits (ICs), or a substrate with metalredistribution lines.

In various embodiments, description is made with reference to figures.However, certain embodiments may be practiced without one or more ofthese specific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions andprocesses, etc., in order to provide a thorough understanding of thepresent invention. In other instances, well-known semiconductorprocesses and manufacturing techniques have not been described inparticular detail in order to not unnecessarily obscure the presentinvention. Reference throughout this specification to “one embodiment,”“an embodiment” or the like means that a particular feature, structure,configuration, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention.Thus, the appearances of the phrase “in one embodiment,” “an embodiment”or the like in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, configurations, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

The terms “over”, “to”, “between” and “on” as used herein may refer to arelative position of one layer with respect to other layers. One layer“over” or “on” another layer or bonded “to” another layer may bedirectly in contact with the other layer or may have one or moreintervening layers. One layer “between” layers may be directly incontact with the layers or may have one or more intervening layers.

The terms “micro” device or “micro” LED structure as used herein mayrefer to the descriptive size of certain devices or structures inaccordance with embodiments of the invention. As used herein, the terms“micro” devices or structures are meant to refer to the scale of 1 to100 μm. However, it is to be appreciated that embodiments of the presentinvention are not necessarily so limited, and that certain aspects ofthe embodiments may be applicable to larger, and possibly smaller sizescales.

In one aspect, without being limited to a particular theory, embodimentsof the invention describe micro device transfer heads and head arrayswhich operate in accordance with principles of electrostatic grippers,using the attraction of opposite charges to pick up micro devices. Inaccordance with embodiments of the present invention, a pull-in voltageis applied to a micro device transfer head in order to generate a grippressure on a micro device and pick up the micro device. For example,the transfer head may include a bipolar electrode configuration.

In one aspect, embodiments of the invention describe a compliant bipolarmicro device transfer head and a method of transfer in which an array ofthe compliant bipolar micro device transfer heads enable improvedcontact with an array of micro devices as compared to an array ofnon-compliant transfer heads. The compliant bipolar micro devicetransfer heads include an array of bipolar silicon electrode pairs thatare deflectable into one or more cavities between a base substrate andthe bipolar silicon electrode pairs. In application, as an array ofcompliant bipolar micro device transfer heads are lowered onto an arrayof micro devices, the deflectable silicon electrodes associated withtaller or contaminated micro devices may deflect more than siliconelectrodes associated with shorter micro devices on a carrier substrate.In this manner, the compliant bipolar micro device transfer heads cancompensate for variations in height of the micro devices. Compensatingfor height variations can result in reduced compressive forces appliedto certain micro devices, leading to protection of the physicalintegrity of the micro devices and transfer head array. Compensating forheight variations can also assist each compliant transfer head to makecontact with each micro device, and ensure that each intended microdevice is picked up. Without the compliant nature of the micro devicetransfer heads an irregular micro device height or a particle on a topsurface of a single micro device could prevent the remainder of thetransfer heads from making contact with the remainder of the microdevices in the array. As a result, an air gap could be formed betweenthose transfer heads and micro devices. With such an air gap, it ispossible that the target applied voltage would not create a sufficientgrip pressure to overcome the air gap, resulting in an incompletepick-up process.

In another aspect, embodiments of the invention describe a manner offorming an array of compliant bipolar micro device transfer heads from acommercially available silicon-on-insulator (SOI) substrate including abase substrate, buried oxide layer, and a top silicon layer. In such anembodiment, a silicon interconnect and an array of electrodes are formedfrom the top silicon layer of the SOI substrate. In an embodiment, abipolar electrostatic transfer head includes a pair of siliconelectrodes, where each silicon electrode includes a mesa structure andan electrode lead. The mesa structures for the pair of siliconelectrodes protrude above their respective silicon interconnects toprovide a localized contact point to pick up a specific micro deviceduring a pick up operation. In this manner, it is not necessary to formpatterned metal electrodes. It has been observed that when patterning ofmetal electrodes and electrode leads using a negative photoresist, forexample, it can be difficult to control exposure of the photoresist atdifferent depths (e.g. along both a top surface and down sidewalls of amesa structure). Peeling of the patterned metal layers has also beenobserved during photoresist removal, potentially affecting operabilityof the transfer heads. In accordance with embodiments of the presentinvention, it is not required to form a patterned metal electrode over amesa structure. Instead, the protruding profile of the mesa structure isformed by patterning the silicon electrode to include a raised portioncorresponding to the mesa structure which protrudes away from the basesubstrate and above the silicon interconnect.

Silicon electrodes prepared in accordance with embodiments of theinvention may include integrally formed mesa structures which aresubstantially taller compared to non-integrally formed mesa structureswith patterned metal electrodes. Photolithography can limit patternedmetal electrode structures to heights of 5-10 μm, whereas siliconelectrode mesa structures can be up to 20-30 μm or taller. The mesastructure height for a silicon electrode structure is limited by theetch aspect ratio and the electrode gap (e.g. the trench between mesastructures for a pair of bipolar silicon electrodes). In an embodiment,aspect ratios of mesa structure height to trench width for siliconelectrode mesa structures can range from 10-20:1. For example, siliconelectrode mesa structures in a bipolar electrode configuration can be 20μm tall separated by a 2 μm trench gap between the mesa structures.Taller electrode structures may also afford larger clearance forcontaminant particles and reduce the effects of stray filed onun-targeted micro devices. When compared to metalized mesa structures,silicon electrodes with integrally formed mesa structures can be morerobust to surface contamination and errors in planar alignment of themicro device transfer head in relation to the micro device carriersubstrate.

In another aspect, embodiments of the invention describe a manner offorming an array of micro device transfer heads from a commerciallyavailable silicon-on-insulator (SOI) substrate which allows for aprocessing sequence with minimal processing steps. The processingsequence does not require metal deposition and patterning steps to formmetal electrodes, which relieves thermal processing constraints andallows for the formation of dielectric and passivation layers by hightemperature thermal oxidation resulting in reduced deposition andpatterning operations. Processing sequences in accordance withembodiments of the invention may incorporate simultaneous etching oroxidation operations of different features, reducing the number of masksrequired during processing.

In another aspect, embodiments of the invention describe a transfer headand transfer head array including vias extending through the basesubstrate from a backside of the base substrate to the patterned siliconlayer for connecting the electrodes with working circuitry of a transferhead assembly. The processing sequence in accordance with embodiments ofthe invention also enables passivation of the vias extending through thebase substrate with high temperature thermal oxide growth.

In yet another aspect, embodiments of the invention describe a mannerfor mass transfer of an array of pre-fabricated micro devices with anarray of compliant transfer heads. For example, the pre-fabricated microdevices may have a specific functionality such as, but not limited to, aLED for light-emission, silicon IC for logic and memory, and galliumarsenide (GaAs) circuits for radio frequency (RF) communications. Insome embodiments, arrays of micro LED devices which are poised for pickup are described as having a 10 μm by 10 μm pitch, or 5 μm by 5 μmpitch. At these densities a 6 inch substrate, for example, canaccommodate approximately 165 million micro LED devices with a 10 μm by10 μm pitch, or approximately 660 million micro LED devices with a 5 μmby 5 μm pitch. A transfer tool including an array of compliant transferheads matching an integer multiple of the pitch of the correspondingarray of micro LED devices can be used to pick up and transfer the arrayof micro LED devices to a receiving substrate. In this manner, it ispossible to integrate and assemble micro LED devices intoheterogeneously integrated systems, including substrates of any sizeranging from micro displays to large area displays, and at high transferrates. For example, a 1 cm by 1 cm array of micro device transfer headscan pick up and transfer more than 100,000 micro devices, with largerarrays of micro device transfer heads being capable of transferring moremicro devices.

Referring now to FIG. 1A, a plan view illustration is provided for aportion of a bipolar micro device transfer head array of single sidedclamped cantilever beam pairs with no joints, and includes views atdifferent depths. In the particular embodiment illustrated, the shadedarea illustrates an arrangement of silicon electrodes and siliconinterconnects as viewed from the top surface of the compliant bipolarmicro device transfer head array. The darker shading illustrates abackside via connection as viewed from the backside surface of thecompliant bipolar micro device transfer head array. In this manner, theplan view illustration provides detail regarding structures which havebeen formed from both sides of the SOI wafer.

As illustrated, the compliant bipolar micro device transfer head array100 includes an array of compliant bipolar transfer heads 102 connectedto an arrangement of silicon trace interconnects 104, and businterconnects 106. As illustrated, bus interconnects 106 may be formedaround a periphery or outside a working area of the compliant bipolartransfer head array including the array of compliant transfer heads 102.In an embodiment, each compliant bipolar transfer head 102 includes apair of silicon electrodes 110, with each silicon electrode 110including a mesa structure 112 and an electrode lead 114 connected to asilicon interconnect 104. As illustrated, each compliant transfer head102 is in the form of a pair of single sided clamped cantilever beamsclamped at opposite sides to silicon trace interconnects 104. The pairof silicon electrodes 110 for each compliant bipolar transfer head 102of the embodiment illustrated in FIG. 1A are not joined, as illustratedby an open joint trench 117 between the pair of mesa structures 112. Inthe embodiment illustrated, the array of mesa structure 112 pairs in thecompliant bipolar micro device transfer head array 100 are arranged withapproximately the same pitch as the micro devices to be picked up, forexample, 10 μm by 10 μm, or 5 μm by 5 μm.

In an embodiment, a plurality of vias 120 are formed through thebackside of the base substrate to the patterned silicon layer to makecontact with interconnects 106 in order to electrically connect thesilicon electrodes 110 with working circuitry of a transfer headassembly. In the embodiment illustrated in FIG. 1A, the interconnect 106on the left side of the illustration may be connected to a first voltagesource V_(A), and the interconnect 106 on the right side of theillustration may be connected to a second voltage source V_(B). Whereeach transfer head 102 is operable as a bipolar transfer head, voltagesources V_(A) and V_(B) may simultaneously apply opposite voltages sothat each of the silicon electrodes 110 in a respective transfer head102 has an opposite voltage.

FIG. 1B is a plan view illustration of a compliant bipolar micro devicetransfer head with a pair of single sided clamped cantilever beams andno joint in accordance with an embodiment of the invention. Asillustrated, the opposing silicon electrodes 110 are clamped at oppositesides to silicon trace interconnects 104. For clarity purposes, only asingle bipolar transfer head 102 is illustrated in FIG. 1B as spanningbetween two silicon trance interconnects 104, though an array of bipolartransfer heads may span between the silicon interconnects 104 inaccordance with embodiments of the invention. The pair of siliconelectrodes 110 for each compliant bipolar transfer head 102 are notjoined, as illustrated by the open joint trench 117 between the pair ofmesa structures 112. In the embodiment illustrated, the joint trench 117is parallel to the silicon interconnects 104. FIG. 1C is across-sectional side view illustration taken along transverse line C-Cof the compliant bipolar micro device transfer head illustrated in FIG.1B in accordance with an embodiment of the invention. In the embodimentillustrated in FIG. 1C, each silicon electrode 110 in a bipolarelectrode configuration extends from a separate silicon interconnect104. FIG. 1D is a cross-sectional side view illustration taken alonglongitudinal line D-D of the compliant bipolar micro device transferhead illustrated in FIG. 1B in accordance with an embodiment of theinvention. As illustrated in FIGS. 1C-1D, both the silicon electrodemesa structures 112 and leads 114 extend over and are deflectable into acavity 136 between the base substrate 130 and the silicon electrode 110.In an embodiment, a single cavity 136 is formed underneath an array ofbipolar silicon electrodes 110 and between two separate siliconinterconnects 104. Referring again to FIG. 1A, a single or multipleseparate cavities 136 can be formed between arrays of siliconinterconnects 104. In an embodiment, cavities 136 are the same cavity.For example, cavity 136 may wrap around silicon interconnect 104 andunderneath the array of silicon electrodes 110. Trenches 116 may also beformed in the patterned silicon layer defining the silicon electrodes110 and silicon interconnects 104, 106 as described in more detail inthe following description. A trench 116 may also be formed in thepatterned silicon layer at an end of a silicon interconnect 104 if acavity 136 does not wrap around the end of the silicon interconnect 104.

Referring now to FIGS. 2A-2B, FIGS. 3A-3B and FIGS. 4A-4B variousdifferent compliant bipolar transfer head array configurations inaccordance with embodiments of the invention are illustratedside-by-side. It is to be understood that while the following variationsare separately illustrated and described, the variations are notnecessarily incompatible with one another, and that the variations maybe combined in any suitable manner in one or more embodiment.

FIGS. 2A-2B are a combination plan view illustration and combinationcross-sectional side view illustration taken along lines V-V, W-W, X-X,Y-Y, and Z-Z from FIG. 1A in accordance with an embodiment of theinvention. FIGS. 3A-3B and FIGS. 4A-4B are combination plan viewillustrations and combination cross-sectional side view illustrationsprepared similarly as those in FIGS. 2A-2B. The combination views arenot representations of the precise relative locations for all of thedifferent features illustrated, rather the combination views combinespecific features at different locations previously identified in FIG.1A in order to more easily represent the particular variations inprocessing sequences. For example, while the combination cross-sectionalside view illustrations show one via 120 corresponding to one siliconelectrode 110, it is clear from FIG. 1A that one via 120 may beelectrically connected with a plurality of silicon electrodes 110 alongone or more interconnects 104. As illustrated, lines W-W and Y-Y arealong vias 120. As illustrated, lines V-V and Z-Z are along one or moretrenches 116 defining the silicon electrodes 110 and siliconinterconnects 104, 106. As illustrated, line X-X is across a bipolartransfer head including a pair of silicon electrodes 110. Referringagain to FIG. 1A, one or more cavities 136 may be formed around andbeneath all silicon electrodes 110, and between interconnects 104, 106.

Referring again to FIGS. 2A-2B, a silicon electrode 110 includes a mesastructure 112 and an electrode lead 114, where the mesa structure 112 isan elevated portion of the silicon electrode 110. A dielectric layer 118may cover a top surface of the pair of silicon electrodes 110.Dielectric layer 118 may also cover a side surface of the mesastructures 112 laterally between the pair of mesa structure 112 for thepair of silicon electrodes 110 in a bipolar transfer head 102. In theembodiment illustrated, each cantilever beam compliant transfer head 102is separated by an open space in joint trench 117, and each siliconelectrode 110 is separately deflectable into cavity 136. A via opening120A may extend through the base substrate 130 from a backside of thebase substrate to the patterned silicon layer 140 where interconnect 106is located. In the particular embodiment illustrated in FIGS. 2A-2B, thevia opening 120A extends through a buried oxide layer 124 and terminatesat a bottom surface of the patterned silicon layer 140 whereinterconnect 106 is located. A passivation layer 132 is formed on thebackside of the base substrate 130, and a passivation layer 133 isformed on side surfaces within the via opening 120A. Where basesubstrate is formed of silicon, the passivation layers 132, 133 insulateelectrical shorting between the vias 120. The buried oxide layer 124also insulates electrical shorting between the silicon electrodes 110,and interconnects 104, 106.

The vias 120 illustrated in FIG. 2A-2B extend through the base substrate130 from a backside of the base substrate to a patterned silicon layer140. In an embodiment, vias 120 contact one or more bus interconnects106 in the patterned silicon layer 140. In other embodiments, vias 120may contact other features or interconnects in the patterned siliconlayer 140. Via 120 along line W-W may be electrically connected to afirst interconnect 106 which is connected to a first voltage sourceV_(A), and via 120 along line Y-Y may be electrically connected to asecond interconnect 106 which is connected to a second voltage sourceV_(B). In the particular embodiment illustrated, via openings 120Aextend through a buried oxide layer 124 and terminate at a bottomsurface of an interconnect 106. A passivation layer 132 is formed on thebackside of the base substrate 130 and on side surfaces within the viaopenings 120A. A conductive layer 122 is formed on the passivation layer133 and is in electrical contact with the bottom surface of aninterconnect 106. In the particular embodiment illustrated, theconductive layers 122 do not completely fill the via openings 120A, andthe conductive layers 122 are physically and electrically separated inorder to prevent shorting between vias 120 connected to differentvoltage sources V_(A), V_(B). In an embodiment, vias 120 which areelectrically connected to the same voltage source may or may not bephysically and electrically connected. For example, a conductive layer122 may span across both vias 120 on the left side of FIG. 1A, and alsobe electrically and physically separated from the via 120 taken alongline Y-Y on the right side of FIG. 1A. In an embodiment, the structureillustrated in FIGS. 2A-2B is formed using a total of six masks.

FIGS. 3A-3B are combination plan view and combination cross-sectionalside view illustrations of a compliant bipolar micro device transferhead including a double sided clamped supported beam and an oxide joint119 between and connecting the pair of silicon electrodes 110, andtopside and backside via openings in accordance with an embodiment ofthe invention. It is to be appreciated, that while an oxide joint 119and topside and backside via openings are shown together in FIGS. 3A-3B,that embodiments of the invention are not so limited, and do not requirean oxide joint 119 together with top side and backside via openings. Asillustrated, in one embodiment the oxide joint 119 is formed between andconnects the mesa structures 112 for the pair of silicon electrodes 110,and the oxide joint 119 is on and in direct contact with the buriedoxide layer 140. Since the oxide joint 119 connects the siliconelectrodes 110, the bipolar electrode assembly illustrated in FIGS.3A-3B is characterized as a supported beam structure spanning betweensilicon interconnects. As illustrated, in one embodiment topside viaopening 120B may be formed over the backside via opening 120A to formvia 120. As will become more apparent in the following description, thetopside via opening 120B may be formed in order make electrical contactwith the silicon interconnects 106 and to form an opening through theburied oxide layer 124 without the lithographic challenges associatedwith not adversely affecting the passivation layer 133 along thesidewalls of via openings 120A. A conductive layer 123 can optionally beformed over the exposed top surface of the silicon interconnects 106 andwithin an interior side surface of the silicon interconnects 106. Inthis manner, partially forming conductive layer 123 over the top surfaceof the silicon interconnect 106 can provide greater surface area forohmic contact with the silicon interconnects 106. Due to the closerproximity of the silicon interconnect 106 to the top surface of the SOIstructure than the backside surface of the SOI structure, in accordancewith some embodiments, it may be more efficient to form conductive layer123 within the interior side surface of interconnect 106 from above thetop surface of the SOI structure as opposed to from the back surface ofthe SOI structure. Conductive layer 123 may be formed from the same ordifferent material from conductive layer 122. Conductive layers 122, 123may form a continuous conductive layer along the via 120 side surfaces.In an embodiment, the structure illustrated in FIGS. 3A-3B is formedusing a total of seven masks.

FIGS. 4A-4B are combination plan view and combination cross-sectionalside view illustrations of a compliant bipolar micro device transferhead including a double sided clamped supported beam and a depositeddielectric layer 126, an oxide joint 119 between and connecting the pairof silicon electrodes 110, and topside and backside via openings inaccordance with an embodiment of the invention. It is to be appreciated,that while a deposited dielectric layer 126, an oxide joint 119, andtopside and backside via openings are shown together in FIGS. 4A-4B,that embodiments of the invention are not so limited, and do not requirea deposited dielectric layer 126 together with an oxide joint 119, andtop side and backside via openings. As illustrated, in one embodiment,dielectric layer 118 may be partially or completely removed. In theparticular embodiment illustrated in FIGS. 4A-4B, the dielectric layer118 is removed from over the mesa structures 112. A second dielectriclayer 126 is formed over the top surface of mesa structures 112 and overthe remaining topography of the transfer head array, which may includeportions of dielectric layer 118. Dielectric layer 126 may also coverany of the oxide joint 119, topside via openings 120B and correspondingconductive layers 123, and may partially or completely fill the topsidevia openings 120B within the silicon interconnects 106. In anembodiment, dielectric layer 126 has a higher dielectric constant and/ordielectric breakdown strength than dielectric layer 118. In anembodiment, dielectric layer 118 is thermally grown SiO₂, and dielectriclayer 126 is atomic layer deposition (ALD) SiO₂, Al₂O₃, Ta₂O₅, or RuO₂.It is to be appreciated, that while FIGS. 4A-4B are illustrated as avariation of FIGS. 3A-3B, that the feature of a dielectric layer 126 canbe combined with the embodiments illustrated in FIGS. 2A-2B. In anembodiment, the structure illustrated in FIGS. 4A-4B is formed using atotal of eight masks.

FIGS. 5A-15B illustrate a method of forming a compliant bipolar microdevice transfer head including an open joint trench between a pair ofsilicon electrodes, and backside via openings in accordance with anembodiment of the invention. Initially, the processing sequence maybegin with a commercially available SOI substrate as illustrated inFIGS. 5A-5B. The SOI substrate may include base substrate 130, topsilicon layer 140, a buried oxide layer 124 between the base substrateand the top silicon layer, and backside passivation layer 132. In anembodiment, base substrate is a (100) silicon handle wafer having athickness of 500 μm+/−50 μm, buried oxide layer 124 is 1 μm+/−0.1 μmthick, and top silicon layer is 7-20 μm+/−0.5 μm thick. The top siliconlayer may also be doped to improve conductivity. For example, aphosphorous dopant concentration of approximately 10¹⁷ cm⁻³ yields aresistivity of less than 0.1 ohm-centimeter. In an embodiment, thebackside passivation layer 132 is a thermal oxide having a thickness upto approximately 2 μm thick, which is the approximate upper limit forthermal oxidation of silicon.

A mask layer 142 may then be formed over the top silicon layer 140, asillustrated in FIGS. 6A-6B. Mask layer 142 may be deposited, oralternatively thermally grown from the top silicon layer 140. In anembodiment, mask layer 142 is a thermally growth SiO₂ layer having athickness of approximately 0.1 μm. In an embodiment, where mask layer142 is thermally growth SiO₂, the mask layer 142 has a thickness whichis significantly less than the thickness of buried oxide (SiO₂) layer124 in order to maintain structural stability for the partiallypatterned SOI structure during removal of the patterned mask layer.

Referring to FIGS. 7A-7B, the mask layer 142 is then patterned to forman array of islands 144 which will correspond to the mesa structures ofthe silicon electrodes. In an embodiment, mask layer is a thermallygrown SiO₂ layer, and islands 144 are formed by applying a positivephotoresist, exposing, and removing undeveloped areas of the photoresistwith a potassium hydroxide (KOH) developer solution. The mask layer 142is then dry etched to form islands 144 using a suitable technique suchas ion milling, plasma etching, reactive ion etching (RIE), or reactiveion beam etching (RIBE), electron cyclotron resonance (ECR), orinductively coupled plasma (ICP), stopping on the silicon layer 140. Ifa high degree of anisotropic etching is not required, a dry plasmaetching technique with a plasma etchant such as CF₄, SF₆ or NF₃ may beused. The patterned photoresist is then removed by O₂ ashing followed bypiranha etch resulting in the structure illustrated in FIGS. 7A-7B.

In an embodiment, backside via openings 120A are then formed in the SOIsubstrate. Initially, as illustrated in FIGS. 8A-8B, the backside viaopenings are formed through the backside passivation layer 132 and basesubstrate 130, stopping on the buried oxide layer 124. In an embodiment,the backside via openings 120A illustrated in FIGS. 8A-8B are formed byapplying a patterned positive photoresist on the backside passivationlayer 132, followed by etching of the exposed passivation layer 132 anddry reactive ion etching (DRIE) of the base substrate 130, stopping onthe buried oxide layer 124. The base substrate 130 may alternatively beetched with a wet etchant such as KOH. However, KOH wet etchant attackssilicon preferentially in the (100) plane, and may product ananisotropic V-etch with tapered sidewalls. DRIE etching may be selectedfor more vertical sidewalls in the backside via openings 120A. Afteretching of the base substrate 130, the patterned positive photoresistcan be removed by O₂ ashing followed by piranha etch resulting in thestructure illustrated in FIGS. 8A-8B.

Referring to FIGS. 9A-10B, the silicon electrodes 110 and interconnects104, 106 are patterned in a two part etching sequence. First, asillustrated in FIGS. 9A-9B the top silicon layer 140 is partially etchedthrough, defining the patterns of the silicon electrodes 110 andinterconnects 104, 106. In an embodiment, this may be accomplished witha thin patterned positive photoresist, DRIE etching approximately 5 μmof a 7-10 μm thick top silicon layer 140 in a timed etch. The patternedpositive photoresist can be removed using O₂ ashing followed by piranhaetch. In accordance with embodiments of the invention, openings in thephotoresist 121 (illustrated in FIG. 9A only) on the edges of FIG. 9Acorrespond to the size of the trenches 116 used to define the siliconelectrodes 110 and interconnects 104, 106, however, the openings in thephotoresist 121 over the islands 144 corresponding to the joint trench117 between silicon electrode mesa structures 112 may be larger than thegap between the islands 144. In this manner, the islands 144 in thepatterned hard mask layer 142 can be used to form silicon electrode mesastructures 112 with higher gap resolution of the joint trench openings117 between mesa structures when compared to using photoresist alone. Inan embodiment, the joint trench 117 openings are at least wide enough togrow a dielectric layer 118 on side surfaces of the adjacent mesastructures 112 and to allow deflection of each silicon electrode 110into the cavity 136. For example, the joint trenches 117 may be 2 μmwide or larger.

Second, as illustrated in FIGS. 10A-10B with islands 144 still present,DRIE etching is continued using islands 144 as a mask to form thesilicon electrodes 110 including the protruding mesa structures 112, andsilicon interconnects 104, 106, stopping on the underlying buried oxidelayer 124. Upon completion of etching the silicon layer 140, a dryetching technique is performed to remove the islands 144, approximately0.1 μm. In an embodiment, where only 0.1 μm of oxide is removed, and theburied oxide 124 is approximately 1.0 μm thick, significantly more than0.1 μm of the exposed buried oxide 124 is not removed. In accordancewith embodiments of the invention, the buried oxide 124 providesstructural stability for the partially patterned SOI structure andsignificantly more than the thickness of the islands 144 is not removedfrom the buried oxide 124 is during removal of the islands 144. Asillustrated in FIG. 10B, the buried oxide layer 124 is exposed in jointtrenches 117 between the silicon electrodes, and trenches 116 around thesilicon electrodes and between the interconnects.

Referring now to FIGS. 11A-11B, the front and back sides of the SOIwafer can then be oxidized in order to passivate the silicon electrodes,silicon interconnects, and backside via opening. In an embodiment, hightemperature wet oxidation may be performed in order to grow anapproximately 1 μm thick oxide layer 118 on the silicon electrodes 110,within joint trench 117 between the mesa structures 112, on the siliconinterconnects 104, 106, and within trenches 116. In locations were theburied oxide layer 124 is already exposed, the buried oxide layer 124thickness may increase or remain the same depending upon thepre-existing thickness. In an embodiment, oxide layer 118 isapproximately the same thickness as buried oxide layer 124. Anapproximately 1 μm thick oxide passivation layer 133 is alsosimultaneously grown within the backside via openings 120A alongsidewalls of the base substrate 130.

Referring now to FIGS. 12A-12B, a thick patterned positive photoresistis applied over the interconnects 104, 106 and silicon electrodes 110,followed by etching of the exposed buried oxide in joint trenches 117and trench areas 137 which will correspond to the locations of cavities136 to be formed. The patterned positive photoresist can be removedusing O₂ ashing followed by piranha etch.

A dry oxide etch using a suitable dry etching technique may then beperformed to create openings in the buried oxide layer 124 within thebackside via openings 120A to expose a bottom surface of the patternedsilicon layer 140 where silicon interconnects 106 are formed, asillustrated in FIGS. 13A-13B. In an embodiment, a thin positivephotoresist is formed over the backside of the SOI wafer and within thebackside via opening 120A and patterned. The buried oxide layer 124 isthen etched to expose a bottom surface of the silicon layer 140. In anembodiment, etching of buried oxide layer 124 is performed with RIE. Asillustrated, the openings in the buried oxide layer 124 are smaller(e.g. smaller diameter or cross-section) than the openings within thebase substrate 130 (including the oxide passivation layer 133). In thismanner, having a smaller opening within the buried oxide layer 124 thanin the base substrate (including oxide passivation layer 133) protectsagainst inadvertently etching through the oxide passivation layer 133,or undercutting the oxide passivation layer 133 and electricallyshorting the backside via 120 with the base substrate 130. Due tolithographic tolerances and resolution capabilities, the openings withinthe buried oxide layer 124 may have a minimum cross-section of greaterthan 10 μm.

Referring now to FIGS. 14A-14B, a patterned conductive layer 122 isformed on the passivation layer 133 within the via openings 120A and inelectrical contact with the bottom surface of the silicon interconnects106. In an embodiment, the patterned conductive layer 122 is formed bysputtering through a shadow mask. In an embodiment, the patternedconductive layer 122 includes a first layer of 500 angstrom thicktitanium (Ti), a middle layer of 500 angstrom thick titanium-tungsten(TiW), and a 1 μm to 2 μm thick outer layer of gold (Au). In anembodiment, the patterned conductive layer 122 makes ohmic contact withthe silicon interconnects 106.

Referring now to FIGS. 15A-15B, one or more cavities 136 may then beetched in the base substrate 130 directly underneath the array ofsilicon electrodes such that the array of silicon electrodes aredeflectable into the one or more cavities. In an embodiment, a separatecavity 136 is formed directly underneath each pair of siliconelectrodes. In an embodiment, a single cavity 136 is formed directlyunderneath the array of silicon electrodes in electrical communicationwith the first and second interconnects 104. In an embodiment, cavities136 are formed with a timed release etch into the base substrate 130which undercuts the electrode leads 114 and mesa structures 112. Forexample, etching may be performed with a fluorine based chemistry suchas XeF₂ or SF₆.

Following the formation of the one or more cavities 136, the SOIsubstrate may then be diced, for example using laser dicing, to form acompliant bipolar transfer head array including an array of complianttransfer heads 102 interconnected with silicon interconnects 104, 106and vias 120 extending through the base substrate 130 from a backside ofthe base substrate to the patterned silicon layer 140 to electricallyconnect the silicon electrodes 110 with working circuitry of a transferhead assembly.

FIG. 16A is a plan view illustration of a compliant bipolar micro devicetransfer head array of double sided clamped supported beams and mesajoints in accordance with an embodiment of the invention. The particularembodiment illustrated in FIG. 16A is similar to the embodimentillustrated in FIG. 1A with one difference being that the pair ofsilicon electrodes 110 for each compliant bipolar transfer head 102 arejoined with an oxide joint 119 between the pair of mesa structures 112.As a result of the oxide joint 119, the pair of silicon electrodes in abipolar micro device transfer head are in the form of a doubled sidedclamed supported beam, which is supported at opposite sides with siliconinterconnects 104. A single cavity 136 may be formed underneath an arrayof transfer heads 102 spanning between a pair of silicon interconnects104. A plurality of cavities 136 may be formed between a plurality ofpairs of silicon interconnects 104 or a single cavity 136 may be formedbetween a plurality of pairs of silicon interconnects 104. Trenches 116may also be formed in the patterned silicon layer defining the siliconelectrodes 110 and silicon interconnects 104, 106.

FIG. 16B is a plan view illustration of a compliant bipolar micro devicetransfer head with a double sided clamped supported beam and mesa jointin accordance with an embodiment of the invention. FIG. 16C is across-sectional side view illustration taken along transverse line C-Cof the compliant bipolar micro device transfer head illustrated in FIG.16B in accordance with an embodiment of the invention. FIG. 16D is across-sectional side view illustration taken along longitudinal line D-Dof the compliant bipolar micro device transfer head illustrated in FIG.16B in accordance with an embodiment of the invention. Similar to theembodiments illustrated in FIGS. 1B-1D, only a single transfer head 102is illustrated in FIG. 16B as spanning between and being supported bytwo silicon trace interconnects 104, though an array of transfer headsmay span between the silicon interconnects 104 in accordance withembodiments of the invention. The pair of silicon electrodes 110 foreach compliant bipolar transfer head 102 are joined with an oxide joint119 between the pair of mesa structures 112. In the embodimentillustrated, the oxide joint 119 is parallel to the siliconinterconnects 104. As illustrated in FIGS. 16C-16D, both the siliconelectrode mesa structures 112 and leads 114 extend over and aredeflectable into a cavity 136 between the base substrate 130 and thesilicon electrode 110. In the embodiment illustrated in FIG. 16D, theoxide joint 119 is on and in direct contact with the buried oxide layer124.

FIGS. 17A-24B illustrate a method of forming a compliant bipolar microdevice transfer head including a double sided clamped supported beam andan oxide joint between and connecting the pair of silicon electrodes,and topside and backside via openings in accordance with an embodimentof the invention. In an embodiment, the processing sequence leading upto FIGS. 17A-17B may be identical to the processing sequence of FIGS.5A-8B with one difference being the distance between islands 144. Asdescribed in further detail in the following description, the patterningof islands 144 corresponds to the mesa structures 112 to be subsequentlyformed. Furthermore, the distance between islands 144 corresponds to thewidth of the oxide joint 119 which is formed between and connects thepair of silicon electrodes 110. Accordingly, since the oxide joint 119connects the pair of silicon electrodes 110 in the double sided clamedsupported beam configuration, the distance between islands 144 in FIGS.17A-17B, may be less than the distance between the islands 144 in FIGS.8A-8B. For example, the distance between islands may be sufficientlysmall to allow for the joint trench 117 to be completely filled withoxide thermally grown from mesa structures 112. For example, jointtrenches 117 may be 2 μm wide or less.

Referring to FIGS. 17A-18B, the silicon electrodes 110 and interconnects104, 106 may be patterned in a two part etching sequence. First, asillustrated in FIGS. 17A-17B the top silicon layer 140 is partiallyetched through, defining the patterns of the silicon electrodes 110 andinterconnects 104, 106. In an embodiment, this may be accomplished witha thin patterned positive photoresist, DRIE etching approximately 5 μmof a 7-10 μm thick top silicon layer 140 in a timed etch. In accordancewith embodiments of the invention, openings in the photoresist 121(illustrated in FIG. 17A only) on the edges of FIG. 17A correspond tothe size of the trenches 116 used to define the silicon electrodes 110and interconnects 104, 106, however, the openings in the photoresist 121over the islands 144 corresponding to the joint trench 117 betweensilicon electrode mesa structures 112 may be larger than the gap betweenthe islands 144. In this manner, the islands 144 in the patterned hardmask layer 142 can be used to form silicon electrode mesa structures 112with higher gap resolution of the joint trench openings 117 between mesastructures when compared to using photoresist alone. In this manner, theislands 144 in the patterned hard mask layer 142 can be used to formsilicon electrode mesa structures 112 with higher gap resolution betweenmesa structures when compared to using photoresist alone, which mayassist in increasing electrode active area and resultant grip pressureacross the array of compliant transfer heads. For example, as microdevice size decreases a narrower gap between mesa structures mayincrease the available electrode space with regard to a micro device tobe picked up. The patterned positive photoresist can be removed using O₂ashing followed by piranha etch.

Second, as illustrated in FIGS. 18A-18B with islands 144 still present,DRIE etching is continued using islands 144 as a mask to form thesilicon electrodes 110 including the protruding mesa structures 112, andinterconnects 104, 106, stopping on the underlying buried oxide layer124. Upon completion of etching the silicon layer 140, a dry etchingtechnique is performed to remove the islands 144, approximately 0.1 μm.In an embodiment, where only 0.1 μm of oxide is removed, and the buriedoxide 124 is approximately 1.0 μm thick, significantly more than 0.1 μmof the exposed buried oxide 124 is not removed. In accordance withembodiments of the invention, the buried oxide 124 provides structuralstability for the partially patterned SOI structure and significantlymore than the thickness of the islands 144 is not removed from theburied oxide 124 is during removal of the islands 144.

Referring now to FIGS. 19A-19B, the front and back sides of the SOIwafer can then be oxidized in order to passivate the silicon electrodes,silicon interconnects, and backside via opening. In an embodiment, hightemperature wet oxidation may be performed in order to grow anapproximately 1 μm thick oxide layer 118 on the silicon electrodes 110,within joint trench 117 between the mesa structures 112, on the siliconinterconnects 104, 106, and within trenches 116. As described above,where oxide layer 118 is grown within and fills joint trench 117, theoxide layer forms oxide joint 119. In an embodiment, oxide joint 119completely fills joint trench 117. In locations were the buried oxidelayer 124 is already exposed, the buried oxide layer 124 thickness mayincrease or remain the same during thermal oxidation depending upon thepre-existing thickness. In an embodiment, oxide layer 118 isapproximately the same thickness as buried oxide layer 124. Anapproximately 1 μm thick oxide passivation layer 133 is alsosimultaneously grown within the backside via openings 120A alongsidewalls of the base substrate 130.

Referring now to FIGS. 20A-20B, openings (which will become part of viaopenings 120B) are formed in the top dielectric layer 118 to expose thepatterned silicon layer 140 at regions of silicon interconnects 106directly above the backside via openings 120A and at trench areas 137where the one or more cavities 136 will be formed. Trench area 137openings are also simultaneously formed in buried oxide layer 124 toexpose the base substrate 130 where the one or more cavities 136 will beformed. Openings may be formed in top dielectric layer 118 and buriedoxide layer 124 with a thick patterned positive photoresist, followed bydry etching of the top dielectric layer 118. The patterned photoresistis then removed by O₂ ashing followed by piranha etch resulting in thestructure in FIGS. 20A-20B. Combining the etching and patterning stepsto form via openings 120B and trench area 137 openings also may reduceprocessing operations and number of masks required.

Referring now to FIGS. 21A-21B, openings are formed in the silicon layer140 and buried oxide layer 124 to form a topside via opening 120B whichconnects with backside via opening 120A. Openings may be formed in thesilicon layer 140 and buried oxide layer 124 by forming a thickpatterned positive photoresist, followed by DRIE of the silicon layer140 stopping on the buried oxide layer 124, followed by RIE through theburied oxide layer 124. The patterned photoresist is then removed by O₂ashing followed by piranha etch resulting in the structure in FIGS.21A-21B. In this manner, forming the openings through the buried oxidelayer 124 when forming the topside via openings 120B may avoid thelithographic challenges associated with forming an opening in the buriedoxide layer 124 from the backside of the SOI structure without adverselyaffecting the passivation layer 133 along the sidewalls of the viaopenings 120A.

A patterned conductive layer 123 may then be formed over the exposed topsurface of the silicon interconnects 106 and within an interior sidesurface of the silicon interconnects 106, as illustrated in FIGS.22A-22B. In this manner, partially forming conductive layer 123 over thetop surface of the silicon interconnect 106 can provide greater surfacearea for ohmic contact with the silicon interconnects 106. Due to thecloser proximity of the silicon interconnect 106 to the top surface ofthe SOI structure than the backside surface of the SOI structure, inaccordance with some embodiments, it may be more efficient to form alayer of conductive layer 123 within the interior side surface ofinterconnect 106 from above the top surface of the SOI structure asopposed to from the back surface of the SOI structure. In an embodiment,the patterned conductive layer 123 is formed by sputtering through ashadow mask. In an embodiment, the patterned conductive layer 123includes a first layer of 500 angstrom thick titanium (Ti), a middlelayer of 500 angstrom thick titanium-tungsten (TiW), and a 1 μm to 2 μmthick outer layer of gold (Au). In an embodiment, the patternedconductive layer 123 makes ohmic contact with the silicon interconnects106.

Referring now to FIGS. 23A-23B, a patterned conductive layer 122 may beformed on the passivation layer 133 within the via openings 120A and inelectrical contact with the patterned conductive layer 123. Conductivelayer 122 may be formed from the same or different material fromconductive layer 123, and may have the same or different thicknesses. Inan embodiment, conductive layer 123 has a thicker layer of gold.

Referring now to FIGS. 24A-24B, one or more cavities 136 may then beetched in the base substrate 130 directly underneath the array ofsilicon electrodes such that the array of silicon electrodes aredeflectable into the one or more cavities. In an embodiment, a separatecavity 136 is formed directly underneath each pair of siliconelectrodes. In an embodiment, a single cavity 136 is formed directlyunderneath the array of silicon electrodes in electrical communicationwith the first and second interconnects 104. In an embodiment, cavities136 are formed with a timed release etch into the base substrate 130which undercuts the electrode leads 114 and mesa structures 112. Forexample, etching may be performed with a fluorine based chemistry suchas XeF₂ or SF₆. In an embodiment, the one or more cavities 136 areapproximately 15 μm deep.

Following the formation of the one or more cavities 136, the SOIsubstrate may then be diced, for example using laser dicing, to form acompliant bipolar transfer head array including an array of complianttransfer heads 102 interconnected with silicon interconnects 104, 106and vias 120 extending through the base substrate 130 from a backside ofthe base substrate to the patterned silicon layer 140, and through thepatterned silicon layer 140, to electrically connect the siliconelectrodes 110 with working circuitry of a transfer head assembly.

FIGS. 25A-30B illustrate a method of forming a compliant bipolar microdevice transfer head including a double sided clamped supported beam anda deposited dielectric layer 126, an oxide joint 119 between andconnecting the pair of silicon electrodes 110, and topside and backsidevia openings in accordance with an embodiment of the invention. In anembodiment, the processing sequence leading up to FIGS. 25A-25B may beidentical to the processing sequence of FIGS. 5A-7B and FIGS. 17A-19B asdescribed above. Referring now to FIGS. 25A-25B, in an embodimentopenings are formed in the top dielectric layer 118 directly above thebackside via openings 120A and directly over the mesa structures 112.

Referring now to FIGS. 25A-25B, openings are formed in the topdielectric layer 118 to expose the mesa structures 112 and oxide joint119 (and optionally portions of electrode leads 114), and openings(which will become part of via openings 120B) are formed in the topdielectric layer 118 directly above the backside via openings 120A.Trench area 137 openings are also simultaneously formed in buried oxidelayer 124 to expose the base substrate 130 where the one or morecavities 136 will be formed. In the particular embodiment illustrated,the oxide joint 119 is not completely removed from between the adjacentmesa structures 112 in a bipolar electrode transfer head 102. Openingsmay be formed in top dielectric layer 118 and buried oxide layer 124with a thick patterned positive photoresist, followed by dry etching ofthe top dielectric layer 118. In an embodiment a timed dry oxide etch isperformed to ensure oxide joint 119 is not completely removed. In anembodiment, top dielectric layer 118 and buried oxide layer 124 haveapproximately the same thickness, and may be completely removed in atimed dry oxide etch while removing less than 0.2 μm of the oxide joint119 thickness. The patterned photoresist is then removed by O₂ ashingfollowed by piranha etch resulting in the structure in FIGS. 25A-25B.Combining the etching and patterning steps to form via openings 120A andtrench area 137 openings also may reduce processing operations andnumber of masks required.

Referring now to FIGS. 26A-26B, in an embodiment, a second dielectriclayer 126 is formed over the top surface including the patterneddielectric layer 118, patterned silicon layer 140 and oxide joint 119,followed by patterning with a thick positive resist and etched. Uponcompletion of etching, the patterned second dielectric layer 126 coversthe mesa structures 112 and may also cover a portion of the electrodeleads 114 and patterned dielectric layer 118. The patterned seconddielectric layer 126 is removed from over the patterned silicon layer140 directly above the backside via openings 120A, and at trench areas137 where the one or more cavities 136 will be formed. In an embodiment,the second dielectric layer may have a higher dielectric constant ordielectric breakdown strength than dielectric layer 118, and has athickness between 0.5 μm-10 μm. For example, the second dielectric layer126 a layer of Al₂O₃, Ta₂O₅, or HfO₂ deposited by atomic layerdeposition (ALD).

Referring now to FIGS. 27A-27B, openings are formed in the silicon layer140 and buried oxide layer 124 to form topside via openings 120B whichconnect with backside via openings 120A. Openings may be formed in thesilicon layer 140 and buried oxide layer 124 by forming a thickpatterned positive photoresist, followed by DRIE of the silicon layer140 stopping on the buried oxide layer 124, followed by RIE through theburied oxide layer 124. The patterned photoresist is then removed by O₂ashing followed by piranha etch resulting in the structure in FIGS.27A-27B. In this manner, forming the openings through the buried oxidelayer 124 when forming the topside via openings 120B may avoid thelithographic challenges associated with forming an opening in the buriedoxide layer 124 from the backside of the SOI structure without adverselyaffecting the passivation layer 133 along the sidewalls of the viaopenings 120A.

A patterned conductive layer 123 is then formed over the exposed topsurface of the silicon interconnects 106 and within an interior sidesurface of the silicon interconnects 106, as illustrated in FIGS.28A-28B. In this manner, partially forming conductive layer 123 over thetop surface of the silicon interconnect 106 can provide greater surfacearea for ohmic contact with the silicon interconnects 106. Due to thecloser proximity of the silicon interconnect 106 to the top surface ofthe SOI structure than the backside surface of the SOI structure, inaccordance with some embodiments, it may be more efficient to form alayer of conductive layer 123 within the interior side surface ofinterconnect 106 from above the top surface of the SOI structure asopposed to from the back surface of the SOI structure. In an embodiment,the patterned conductive layer 123 is formed by sputtering through ashadow mask. In an embodiment, the patterned conductive layer 123includes a first layer of 500 angstrom thick titanium (Ti), a middlelayer of 500 angstrom thick titanium-tungsten (TiW), and a 1 μm to 2 μmthick outer layer of gold (Au). In an embodiment, the patternedconductive layer 123 makes ohmic contact with the silicon interconnects106.

A patterned conductive layer 122 may be formed on the passivation layer133 within the via openings 120A and in electrical contact with thepatterned conductive layer 123 as illustrated in FIGS. 29A-29B.Conductive layer 122 may be formed from the same or different materialfrom conductive layer 123, and may have the same or differentthicknesses. In an embodiment, conductive layer 123 has a thicker layerof gold. Conductive layers 122, 123 may form a continuous conductivelayer along the via 120 side surfaces.

Referring now to FIGS. 30A-30B, one or more cavities 136 may then beetched in the base substrate 130 directly underneath the array ofsilicon electrodes such that the array of silicon electrodes aredeflectable into the one or more cavities. In an embodiment, a separatecavity 136 is formed directly underneath each pair of siliconelectrodes. In an embodiment, a single cavity 136 is formed directlyunderneath the array of silicon electrodes in electrical communicationwith the first and second interconnects 104. In an embodiment, cavities136 are formed with a timed release etch into the base substrate 130which undercuts the electrode leads 114 and mesa structures 112. Forexample, etching may be performed with a fluorine based chemistry suchas XeF₂ or SF₆. In an embodiment, the one or more cavities 136 areapproximately 15 μm deep.

Following the formation of the one or more cavities 136, the SOIsubstrate may then be diced, for example using laser dicing, to form acompliant bipolar transfer head array including an array of complianttransfer heads 102 interconnected with silicon interconnects 104, 106and vias 120 extending through the base substrate 130 from a backside ofthe base substrate to the patterned silicon layer 140, and through thepatterned silicon layer 140, to electrically connect the siliconelectrodes 110 with working circuitry of a transfer head assembly.

FIGS. 31-37 illustrate various modifications of compliant bipolar microdevice transfer heads spanning between silicon interconnects 104 inaccordance with embodiments of the invention. While FIGS. 31-37 areillustrated separately from the processing sequences illustrated above,it is to be appreciated that many of the various modifications describedwith respect to FIGS. 31-37 can be implemented into the processingsequences previously described.

FIG. 31 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with cantilever beam and continuous joint in accordancewith an embodiment of the invention. As illustrated, a silicon electrodecantilever beam may include a pair of silicon electrode leads 114extending from two silicon interconnects 104, and pair mesa structures112 separated by a continuous oxide joint 117 which is on and in directcontact with the buried oxide layer 124 and extends in a longitudinallength of the cantilever beam parallel to the pair of siliconinterconnects 104. In such an embodiment, the oxide joint 117electrically insulates the pair of silicon electrodes in the bipolarelectrode configuration along a longitudinal length of the cantileverbeam along both the pair silicon electrode leads 114 and pair of mesastructures 112. As illustrated, the silicon electrode leads 114 mayinclude a bend 115 (illustrated as a 90 degree bend).

FIG. 32 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with cantilever beam and mesa joint in accordance with anembodiment of the invention. As illustrated, a silicon electrodecantilever beam may include a pair of silicon electrode leads 114extending from two silicon interconnects 104, and pair mesa structures112 separated by a mesa oxide joint 117 which is on and in directcontact with the buried oxide layer 124 and extends in a longitudinallength of the cantilever beam parallel to the pair of siliconinterconnects 104. In such an embodiment, the oxide joint 117electrically insulates the pair of silicon electrodes in the bipolarelectrode configuration along a longitudinal length of the cantileverbeam along the pair of mesa structures 112. A illustrated, the pair ofsilicon electrode leads 114 are physically separated by patterning andmay include a bend 115 (illustrated as a 90 degree bend).

FIG. 33 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with double sided clamped beam and continuous joint inaccordance with an embodiment of the invention. As illustrated, asilicon electrode double sided clamped beam may include a pair of bentsilicon electrode leads 114 extending from two silicon interconnects104, and pair mesa structures 112 separated by a continuous oxide joint117 which is on and in direct contact with the buried oxide layer 124and extends in a longitudinal length of the cantilever beam parallel tothe pair of silicon interconnects 104. In such an embodiment, the oxidejoint 117 electrically insulates the pair of silicon electrodes in thebipolar electrode configuration along a longitudinal length of thedouble sided clamped beam along both the pair silicon electrode leads114 and pair of mesa structures 112. As illustrated, the siliconelectrode leads 114 may each include bends 115 (illustrated as 90 degreebends) at proximal and distal locations where the electrode leads extendfrom the silicon interconnects 104.

FIG. 34 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with a double sided clamped beam including a pair ofsilicon electrodes with double bends and a mesa joint in accordance withan embodiment of the invention. As illustrated, a silicon electrodedouble sided clamped beam may include a pair of silicon electrode leads114 extending from two silicon interconnects 104, each lead 114 with adouble bend 115, and pair mesa structures 112 separated by a mesa oxidejoint 117 which is on and in direct contact with the buried oxide layer124 and extends in a transverse width of the double sided clamped beamparallel to the pair of silicon interconnects 104. In such anembodiment, the oxide joint 117 electrically insulates the pair ofsilicon electrodes in the bipolar electrode configuration along atransverse width of the cantilever beam between the pair of mesastructures 112, and the pair of silicon electrode leads 114 arephysically separated by patterning. In the embodiment illustrated, eachelectrode lead 114 is split, so that the beam configuration assumes an8-shape configuration with the silicon electrode leads 114.

FIG. 35 is a plan view illustration and cross-sectional side viewillustration taken along line A-A of a compliant bipolar micro devicetransfer head with a double sided clamped beam including a pair ofsilicon electrodes with single bends and a mesa joint in accordance withan embodiment of the invention. As illustrated, a silicon electrodedouble sided clamped beam may include a pair of silicon electrode leads114 extending from two silicon interconnects 104, each lead 114 with asingle bend 115, and pair mesa structures 112 separated by a mesa oxidejoint 117 which is on and in direct contact with the buried oxide layer124 and extends in a transverse width of the double sided clamped beamperpendicular to the pair of silicon interconnects 104. In such anembodiment, the oxide joint 117 electrically insulates the pair ofsilicon electrodes in the bipolar electrode configuration along atransverse width of the double sided clamped beam between the pair ofmesa structures 112, and the pair of silicon electrode leads 114 arephysically separated by patterning.

FIGS. 36-37 are plan view illustrations and cross-sectional side viewillustrations taken along line A-A of a compliant bipolar micro devicetransfer head with a double sided clamped beam including a pair ofsilicon electrodes with double bends and a mesa joint in accordance withan embodiment of the invention. As illustrated, a silicon electrodedouble sided clamped beam may include a pair of silicon electrode leads114 each with a double bend 115, and pair mesa structures 112 separatedby a mesa oxide joint 117 which is on and in direct contact with theburied oxide layer 124 and extends in a transverse width of the doublesided clamped beam parallel to the pair of silicon interconnects 104. Insuch an embodiment, the oxide joint 117 electrically insulates the pairof silicon electrodes in the bipolar electrode configuration along atransverse width of the double sided clamped beam between the pair ofmesa structures 112. In the particular embodiment illustrated in FIG.36, the beam is in a W-shape configuration. In the particular embodimentillustrated in FIG. 37, the beam is in an S-shape configuration.

In accordance with embodiments of the invention, the dielectric layer118 or 126 covering the mesa structures 112 has a suitable thickness anddielectric constant for achieving the required grip pressure for themicro device transfer head, and sufficient dielectric strength to notbreak down at the operating voltage. FIG. 38 is a flow chartillustrating a method of picking up and transferring an array of microdevices from a carrier substrate to a receiving substrate in accordancewith an embodiment of the invention. At operation 3810 an array ofcompliant transfer heads is positioned over an array of micro devices ona carrier substrate. FIG. 39 is a cross-sectional side view illustrationof an array of compliant bipolar micro device transfer heads 102positioned over an array of micro devices on a carrier substrate 200 inaccordance with an embodiment of the invention. At operation 3820 thearray of micro devices are contacted with the array of complianttransfer heads. In an alternative embodiment, the array of complianttransfer heads is positioned over the array of micro devices with asuitable air gap separating them which does not significantly affect thegrip pressure, for example, 1 nm to 10 nm. FIG. 40 is a cross-sectionalside view illustration of an array of compliant bipolar micro devicetransfer heads 102 in contact with an array of micro devices 202 inaccordance with an embodiment of the invention. As illustrated, thepitch of the array of compliant transfer heads 102 is an integermultiple of the pitch of the array of micro devices 202. At operation3830 a voltage is applied to the array of compliant transfer heads 102.The voltage may be applied from the working circuitry within a complianttransfer head assembly 160 in electrical connection with the array ofcompliant transfer heads through vias 120. At operation 3840 the arrayof micro devices is picked up with the array of compliant transferheads. FIG. 41 is a cross-sectional side view illustration of an arrayof compliant transfer heads 102 picking up an array of micro devices 202in accordance with an embodiment of the invention. At operation 3850 thearray of micro devices is then released onto a receiving substrate. Forexample, the receiving substrate may be, but is not limited to, adisplay substrate, a lighting substrate, a substrate with functionaldevices such as transistors or ICs, or a substrate with metalredistribution lines. FIG. 42 is a cross-sectional side viewillustration of an array of micro devices 202 released onto a receivingsubstrate 300 in accordance with an embodiment of the invention.

While operations 3810-3850 have been illustrated sequentially in FIG.38, it is to be appreciated that embodiments are not so limited and thatadditional operations may be performed and certain operations may beperformed in a different sequence. For example, in one embodiment, anoperation is performed to create a phase change in a bonding layerconnecting the micro device to the carrier substrate prior to or whilepicking up the micro device. For example, the bonding layer may have aliquidus temperature less than 350° C., or more specifically less than200° C. The bonding layer may be formed of a material which providesadhesion to the carrier substrate, yet also a medium from which themicro device is readily releasable. In an embodiment, the bonding layeris a material such as indium or an indium alloy. If a portion of thebonding layer is picked up with the micro device, additional operationscan be performed to control the phase of the portion of the bondinglayer during subsequent processing. For example, heat can be applied tothe bonding layer from a heat source located within the transfer headassembly 160, carrier substrate 200, and/or receiving substrate 300.

Furthermore, operation 3830 of applying the voltage to create a grippressure on the micro devices can be performed in various orders. Forexample, the voltage can be applied prior to contacting the array ofmicro devices with the array of compliant transfer heads, whilecontacting the micro devices with the array of compliant transfer heads,or after contacting the micro devices with the array of complianttransfer heads. The voltage may also be applied prior to, while, orafter creating a phase change in the bonding layer.

Where the compliant transfer heads 102 include bipolar siliconelectrodes, an alternating voltage is applied across the pair of siliconelectrodes in each compliant transfer head 102 so that at a particularpoint when a negative voltage is applied to one silicon electrode, apositive voltage is applied to the other silicon electrode in the pair,and vice versa to create the pickup pressure. Releasing the microdevices from the compliant transfer heads 102 may be accomplished with avaried of methods including turning off the voltage sources, loweringthe voltage across the pair of silicon electrodes, changing a waveformof the AC voltage, and grounding the voltage sources. Release may alsobe accomplished by discharge associated with placing the micro deviceson the receiving substrate.

In utilizing the various aspects of this invention, it would becomeapparent to one skilled in the art that combinations or variations ofthe above embodiments are possible for forming a compliant bipolar microdevice transfer head and head array, and for transferring a micro deviceand micro device array. Although the present invention has beendescribed in language specific to structural features and/ormethodological acts, it is to be understood that the invention definedin the appended claims is not necessarily limited to the specificfeatures or acts described. The specific features and acts disclosed areinstead to be understood as particularly graceful implementations of theclaimed invention useful for illustrating the present invention.

What is claimed is:
 1. A transfer head array comprising: a basesubstrate; and an array of compliant transfer heads; wherein eachcompliant transfer head comprises a double sided clamped beam thatextends across a cavity from a first side of the cavity to a second sideof the cavity, and a mesa structure supported by the double sidedclamped beam, wherein the mesa structure protrudes from the double sidedclamped beam over the cavity to provide a localized contact point forthe compliant transfer head; and wherein each double sided clamped beamcomprises: a first beam length extending from the first side of thecavity; a second beam length extending from the second side of thecavity; wherein the first beam length is joined to the second beamlength over the cavity; a first 180 degree bend along the first beamlength that extends from the first side of the cavity to the second beamlength; and a second 180 degree bend along the second beam length thatextends from the second side of the cavity to the second beam length. 2.The transfer head array of claim 1, wherein the first 180 degree bendincludes a first pair of bends.
 3. The transfer had array of claim 2,wherein the second 180 degree bend includes a second pair of bends. 4.The transfer head array of claim 1, wherein the double sided clampedbeam is in an S-shape configuration, with a first end of the S-shapeconfiguration extending from the first side of the cavity and a secondend of the S-shape configuration extending from the second side of thecavity.
 5. The transfer head array of claim 1, wherein the double sidedclamped beam is in a W-shape configuration, with a first end of theW-shape configuration extending from the first side of the cavity and asecond end of the W-shape configuration extending from the second sideof the cavity.
 6. The transfer head array of claim 1, further comprisinga dielectric layer covering the mesa structure of each complianttransfer head.
 7. The transfer head array of claim 1, wherein eachdouble sided clamped beam extends across the same cavity.
 8. Thetransfer head array of claim 1, wherein each double sided clamped beamextends across a separate cavity.
 9. The transfer head array of claim 1,wherein each double sided clamped beam comprises a pair of protrudingmesa structures over the cavity to provide the localized contact pointfor the compliant transfer head, the pair of protruding mesa structuresincluding the protruding mesa structure and a second protruding mesastructure, wherein the protruding mesa structure protrudes from thefirst beam length and the second protruding mesa structure protrudesfrom the second beam length.
 10. The transfer head array of claim 9,wherein the first 180 degree bend includes a first pair of bends. 11.The transfer had array of claim 10, wherein the second 180 degree bendincludes a second pair of bends.
 12. The transfer head array of claim 9,wherein the double sided clamped beam is in an S-shape configuration,with a first end of the S-shape configuration extending from the firstside of the cavity and a second end of the S-shape configurationextending from the second side of the cavity.
 13. The transfer headarray of claim 9, wherein the double sided clamped beam is in a W-shapeconfiguration, with a first end of the W-shape configuration extendingfrom the first side of the cavity and a second end of the W-shapeconfiguration extending from the second side of the cavity.
 14. Thetransfer head array of claim 9, further comprising a dielectric layercovering the pair of mesa structures of each compliant transfer head.15. The transfer head array of claim 14, wherein the dielectric layer isformed between the pair of mesa structures of each compliant transferhead.
 16. The transfer head array of claim 14, further comprising adielectric joint between the protruding mesa structure and the secondprotruding mesa structure of each compliant transfer head.
 17. Thetransfer head array of claim 16, wherein the dielectric layer is formedover the dielectric joint.
 18. The transfer head array of claim 17,wherein the dielectric layer is formed of a different material than thedielectric joint.
 19. The transfer head array of claim 9, wherein eachdouble sided clamped beam extends across the same cavity.
 20. Thetransfer head array of claim 9, wherein each double sided clamped beamextends across a separate cavity.